The present invention relates generally to processes and systems for converting lower molecular weight alkanes to higher molecular weight hydrocarbons and, more particularly, in one or more embodiments, to processes for converting lower molecular weight alkanes that include fractionation of brominated hydrocarbons, wherein the brominated hydrocarbons are framed by reaction of the lower molecular weight alkanes with bromine.
Natural gas, which is primarily composed of methane and other light alkanes, has been discovered in large quantities throughout the world. In the United States, the latest proved natural gas reserves are 6,731 billion standard cubic meter (238 trillion standard cubic feet) in 2010, which makes the United States a top-five country in natural gas abundance. Natural gas is generally a cleaner energy source than crude oil. It is normally heavy sulfur-free and contains none or a minimum amount of heavy metals and non-reacting heavy hydrocarbons. For a given amount of heat energy, burning natural gas produces about half as much carbon dioxide as coal.
However, the transportation, storage and distribution of natural gas in a gaseous form are much less favorable than those of crude oil making it more difficult to be a substitute as the predominant energy source. Converting natural gas to higher molecular weight hydrocarbons, which, due to their higher density and value, are able to be more economically transported, can significantly aid the development of natural gas reserves, particularly the stranded remote natural gas reserves.
One technique for converting natural gas to higher molecular weight hydrocarbons is a bromine-based process. In general, the bromine-based process may include several basic steps, as listed below.                (1) Bromination: Reacting bromine with lower molecular weight alkanes to produce alkyl bromides and hydrogen bromide (HBr).        (2) Alkyl Bromides Conversion: Reacting the alkyl bromides over a suitable catalyst under sufficient conditions to produce HBr, methane (C1), light end hydrocarbons (C2-C4) and heavy end hydrocarbons (C5+).        (3) HBr Recovery: Recovering HBr produced in both steps (1) and (2) by one of several processes, e.g., absorbing HBr and neutralizing the resulting hydrobromic acid with an aqueous solution of partially oxidized metal bromide salts (as metal oxides/oxy-bromides/bromides) to produce metal bromide salt and water in an aqueous solution; reacting HBr with metal oxide; or absorbing HBr into water using a packed tower or other contacting device.        (4) Bromine Regeneration: Reacting the bromide recovered in step (3) with oxygen or air to yield bromine and treating it sufficiently for recycle to step (1).        (5) Product Recovery: Fractionating by distillation and cryogenic distillation (demethanizer) the hydrocarbon mixtures contained in the effluent from step (2) and then separated from HBr in step (3) into methane, light end hydrocarbons, and heavy end hydrocarbons. The methane can be compressed for recycle to step (1). The light end hydrocarbons (C2-C4) may be, for example, salable as a product or cracked to produce light olefins. The heavy end hydrocarbons (C5+) may be used, for example, for further petrochemical or fuel processing.        
In the bromine-based processes, monobrominated alkanes created during bromination may be desirable as the predominant reactant species for the subsequent alkyl bromide conversion. Polybrominated alkanes are known to adversely affect the selectivity profiles of the higher molecular weight hydrocarbons produced during the alkyl bromide conversion and, more importantly, promote the formation of coke which can deposit on the catalyst, block the active sites, and cause rapid catalyst deactivation. The higher selectivity of polybrominated alkanes can also lower the utilization efficiency of bromine, requiring a higher circulating flow of bromine which can correspond to a higher cost in recovering HBr and regenerating recyclable bromine.
To achieve higher selectivity of monobrominated alkanes and reduce the formation of bromination carbon/soot, a large excess of methane or large methane-to-bromine ratio can be used. In the case of the bromination of methane, a methane-to-bromine ratio of about 6:1 can be used to increase the selectivity to mono-bromomethane (CH3Br) to average approximately 88% depending on other reaction conditions. If a lower methane-to-bromine ratio of approximately 2.6:1 is utilized, selectivity of CH3Br may fall to the range of approximately 65-75% depending, for example, on other reaction conditions. If a methane-to-bromine ratio significantly less than 2.5:1 is utilized, unacceptably low selectivity to CH3Br occurs, and, moreover, significant formation of undesirable di-bromomethane (CH2Br2), tri-bromomethane (CHBr3), and carbon soot is observed. However, the large methane-to-bromine ratio can be problematic, in that the large excess methane represents a large recycle stream circulating throughout the entire system. For example, the pressure drop of the process gas between the feed to bromination in step (1) and the recycle methane from product recovery in the step (5) can be large, resulting in a high cost of compression for the recycle gas.
In alkyl bromide conversion, the exothermic coupling reaction may be carried out in a fixed-bed, fluidized-bed or other suitable reactor in the presence of suitable catalysts under sufficient conditions (e.g., 150-600° C., 1-80 bars). The catalyst may have to undergo decoking periodically or continuously to maintain adequate performance. In some instances, a fluidized-bed reactor may be considered to be advantageous for the coupling reaction, particularly for commercial scale of operation, as it should allow for continuous removal of coke and regeneration of the spent catalyst without requiring daily shutdowns and expensive cyclic operation. The fluidized-bed configuration should also facilitate removal of reaction heat and provide a steady selectivity to product composition. However, the fluidized-bed reactor for this particular application may be a very costly item to design and construct as it may have to deal with a high density gas due to the large amount of higher molecular weight bromides contained in the reactor feed (in the forms of HBr and alkyl bromides). Elevated operating pressure, 20-50 bars, may be required to minimize the recompression cost of recycle methane, which, however, will further increase the density of the gases in the synthesis reactor, resulting in a large diameter reactor with heavy wall thickness. In some instances, the catalyst deactivation rate can be lowered by feeding none or the minimum amount of polybromides to the coupling reactor and, thus, the fixed bed configuration may be preferentially selected over fluidized bed.
In product recovery, fresh feed gas may be required to replace the lower molecular weight alkanes converted to products. The fresh feed gas stream containing, for example, primarily methane may necessitate sufficient treating to remove excessive amounts of ethane and higher hydrocarbons prior to being combined with bromine and reacted in a bromination reactor. The feed gas stream may or may not mix with the hydrocarbon mixture exiting HBr recovery prior to receiving such treating. While some ethane and higher hydrocarbons may be tolerated in the bromination reactor, due to the much higher bromination rate of the higher hydrocarbons than that of methane, higher concentrations of the higher hydrocarbon impurities may easily over-brominate and, thus, may result in the rapid formation of carbon-containing coke-like solids, which can cause yield loss and reduced process reliability by fouling and plugging the reactor as well as the downstream units. However, the removal of ethane and higher hydrocarbons from the methane by such means as adsorption or cryogenic distillation can be costly. The cost is higher when both the recycle methane and the fresh feed gas stream require the removal of ethane and higher hydrocarbons. The cost is even higher when high methane-to-bromine ratios are used in the bromination, leading to a large flow rate of recycle methane.
Thus, although progress has been made in the conversion of lower molecular weight alkanes to higher molecular weight hydrocarbons, there remains a need for processes that are more efficient, economic, and safe to operate.